1. Field of the Invention
This invention relates to an electro-optic waveguide device of the kind appropriate inter alia for optical beam steering applications.
2. Discussion of Prior Art
Optical beam steering devices based on electro-mechanical movements are well known in the prior art. A typical arrangement comprises a mirror mounted on a current-actuates moving coil. Electro-mechanical systems are inherently limited to low response frequencies up to the order of 1 kHz.
Acousto-optic beam steering devices are also known, such as for example that described by Suhara, Nozaki and Nishihara in the Proceedings of the IVth European Conference on Integrated Optics. This comprises a Ti-doped LiNbO.sub.3 optical waveguide having an interdigital acoustic transducer and a light focussing diffraction grating coupler its upper surface. The grating coupler is curved and has varying (chirped) spatial frequency to provide light output focussing. A radio frequency (RF) signal is applied to the transducer, which produces surface acoustic waves in the waveguide transverse to the light propagation direction. The acoustic waves modulate the waveguide refractive index and thereby interact with light propagation. The output focus from the grating coupler is raster scanned or beam steered by frequency sweeping the RF signal applied to the transducer, deflection angle being approximately proportional to frequency. The transducer had a center frequency of about 500 MHz and a bandwidth of 330 MHz. This beam steering apparatus is of high optical resolution. However, the maximum rate at which the output beam can be steered depends on the rate of propagation of acoustic waves from the transducer across the waveguide, and is in the order of 1 MHz. Moreover, frequency swept RF signal sources are expensive, and are difficult to interface with digital electronic circuitry. In order to achieve beam positioning in response to digital signals, it would be necessary to provide circuitry for converting the signals into RF frequencies within the acoustic transducer bandwidth.
An electro-optic beam steering device is described by R A Meyer in Applied Optics, Vol. 11, pages 613-616, March 1972. It comprises an LiTaO.sub.3 crystal in the form of a rectangular block 0.1 mm thick, 23 mm wide and 15 mm long. One 23 mm.times.15 mm fact of the crystal bears 46 parallel electrodes each 0.2 mm wide and with a center-to-center spacing of 0.5 mm. The electrodes length dimensions are parallel to that of the crystal. Parallel light is input to one 23 mm.times.0.1 mm crystal face and propagates parallel to electrode length. The crystal has electro-optic properties, and consequently its refractive index is a function of electrode voltage. Light output from the crystal is variable in optical phase in accordance with electrode voltage also. The light output form a crystal region immediately beneath the center of an electrode depends on electrode voltage. Because of electrical field non-uniformity at electrode edges and in inter-electrode regions, light propagating in corresponding parts of the crystal is non-uniformly phase modulated and changes its polarization state. The crystal is masked to restrict light output to central areas under electrodes. The mask comprises a linear array of 100 .mu.m square apertures.
Meyer does not provide easily accessible information on the beam steering performance of his device, but it would appear that it is capable of steering a beam through about 0.2.degree.. The limit of beam steering is set by the distance between grating lobes, these being individual diffraction maxima within the main lobe of a diffraction pattern arising form one element or mask aperture alone. The angular beam steering limit is inconveniently small. To obtain a useful degree of linear shift in beam position, a light reception surface would require location at a substantial distance from the device. Furthermore, the device is of inconveniently large dimensions, of the order of centimeters in extent. It is therefore a bulk optical component. It is unsuitable either for production by integrated techniques or for incorporation on a single semiconductor wafer with other electro-optic devices and integrated circuits.
Electro-optic waveguide devices for beam steering applications are described in published European Patent Application EPA 0130859 and British Patent No. GB 1592050. Each of these describes forming an array of individual optical waveguides in a block of electro-optic material. The waveguides are furnished with respective electrodes. The optical phase of light emergent from each waveguide is controlled by electrode voltage, since variation in voltage produces variation in waveguide refractive index and optical path length by virtue of the electro-optic properties of the block. The waveguide outputs constitute an array of coherent light sources with controllable phase relative to one another, and act as a set of diffraction apertures. They consequently provide a far field diffraction pattern like a diffraction grating, the pattern being formed at a distance from the waveguide outputs sufficiently large to produce overlap of their individual beams.
EPA 0130859 envisages formation of the waveguide array by diffusion of titanium into lithium niobate for operation in the visible and near infrared. The use of Ga.sub.x Al.sub.1-x As is also mentioned. Waveguides are diffused into a block, which forms one common electrode. Each waveguide has a respective overlying second electrode. It is envisaged that about one hundred waveguides (91-101) would be required each longer than the corresponding second electrode. The (waveguide/electrode) interaction length would be 40 mm, and the electrode control voltage would be .+-.50 V. Assuming that FIG. 1 of this document is drawn approximately to scale, waveguides over 90 mm long spaced apart by over 17 mm are required. An array of one hundred waveguides would therefore be more than 1700 mm across, well over 1 meter. These parameters are calculated by scaling in proportion to electrode length, the only quoted size factor. The far field diffraction pattern can be shown to be formed at a minimum distance of between ten and one hundred times the width of a waveguide array of this kind, the distance varying with individual waveguide size. Consequently, insofar as it can be ascertained, EPA 0130859 appears to be disclosing a GaAlAs device in the order of meters in extent, and which forms a far field diffraction pattern tens of meters distant. This is far too large to be useful for the purposes of manufacturing integrated electro-optic circuits. In order to be compatible with conventional lithographic semiconductor processing, an electro-optic semiconductor device must be smaller in extent than 10 cm, the diameter of a typical semiconductor wafer. Furthermore, where the wafer is required to accommodate a waveguide array together with other components such as a light source and array outputs detectors, both the array and its far field diffraction pattern must be accommodated within the wafer dimensions. This is clearly quite impracticable with the EPA 0130859 device, since it is orders of magnitude too large. GB 1592050 discloses an electro-optic waveguide array formed by diffusing titanium into lithium niobate. The array has twenty optically discrete waveguides each 18 mm long and 8 .mu.wide with a center-to-center spacing or pitch of 40 .mu.m. Bias electrodes are arranged between adjacent waveguides, ie in the plane of the array. No performance figures are quoted. However, calculations indicate that, at a wavelength of the 1.06 .mu.m, the far field diffraction pattern is not properly formed at a distance in air of 10 cm from the end of the device. This distance scales approximately with refractive index, and is therefore n times greater in a material of refractive index n. Prisms are employed for light input to and output from the device, so the device incorporates bulk optical components unsuitable for integrated optics purposes. GB 1592050 mentions the possibility of using GaAs to form the waveguide array. However, it does not address the problem of the physical size of a GaAs device. As is well known, the refractive index change per unit electric field in lithium niobate is more than ten times that in GaAs. Consequently, to obtain the beam steering properties, a GaAs device constructed in accordance with GB 1592050 would require waveguides about 20 cm long, ie twice the size of a conventional GaAs semiconductor wafer. The far field diffraction pattern in a GaAs medium would be over 30 cm from the waveguide ends. Integration of the device with other components such as deflected beam detectors would therefore require a semiconductor wafer over 50 cm in diameter, five times the diameter and twenty-five times the area of a conventional waver.
GB 1592050 does not quote an operating wavelength, but lithium niobate is considered to be suitable for visible and near infrared use (see eg EPA 0130859). This implies a maximum free space operating wavelength of about 1 .mu.m. The quoted device dimensions consequently correspond to waveguides at least 8.lambda. wide with a center to center spacing of at least 40.lambda., where .lambda. is the free space operating wavelength. In the center of the visible region, these parameters would be 16.lambda. and 80.lambda. respectively. The waveguide spacing is designed to ensure that there is no interaction of light in adjacent waveguides; ie the waveguides are required to be optically discrete. The device has a switching speed of the order of nanoseconds, which implies an operating frequency in the order of hundreds of MHz. At an operating wavelength of 1.06 .mu.m, calculations show that the device produces an angular separation between adjacent diffraction maxima of about 1.degree.. The device consequently has a beam scanning capability limited to this angle if ambiguity is to be avoided.
EPA 0130859 and GB 1592050 both suffer from the problem of electric field non-uniformity perpendicular to the light propagation direction. Both employ waveguides formed by diffusion into a block, and the former employs the block as one electrode and overlying planar metal as the other. The latter discloses electrodes either side of each waveguide on the block surface, across which the electric field appears. The electric field consequently diminishes with depth into the block. Neither of these electrode configurations appears capable of providing uniform electric field in the waveguides as required for uniform electro-optically induced phase change.